Yes you can screw up an injection. c-bino posted this pic (and there are many like them) about glute injections
The main thing you want to be concerned about with glutes especially is the sciatic nerve.
But as far as missing the muscle goes, that'd be really tricky thing to do so long as you aspirate. There are good places and bad places to inject on each muscle, and the spotinjections page handles those very well. Don't put too much worry on the angles, just inject at what angle seems best to you. Usually delts, bi's, tri's I shoot at about 90*. Glutes and pecs I shoot at about 45*-70*. Traps I shoot at about 45* and lats I shoot at about 90*. But don't spend too much time trying to get the angles perfect, you just need experience and it will all come together for you.